1. Field of the Invention
The present invention relates to a centrifugal pump casing, and especially, to a centrifugal pump casing used in a centrifugal pump having a small size and capacity.
2. Prior Art
In general, a centrifugal pump rotatably accommodating an impeller in its casing shell is well known.
Recently, in a pump of this type, it has been proposed to integrally form the casing shell by means of deep drawing of a steel plate by using a press machine.
As shown in FIG. 5, for example, a casing shell 23 is integrally formed by means of pressing work, which has as a basic circle an inner peripheral circle 22 having a diameter slightly greater than the outer peripheral diameter 21 of the impeller 20, and the outer peripheral wall of the casing shell 23 is integrally formed with a bulged portion 23a bulging radially outwardly from the outer peripheral wall of the casing shell. The bulged portion extends from a winding starting point b of the casing shell in the circumferential direction to an end point c while its height gradually increases in the direction from the winding starting point to the end point. And at the end point c, where the bulge height of the bulged portion 23 is maximum, a discharge nozzle 25 is welded to the casing shell.
According to this arrangement, the inside of the casing shell 23 is formed with a volute room A which has a cross-sectional area gradually increasing in the direction of the fluid flow direction. The end point of the volute room A is provided with guide 26 having a projecting shape for decreasing the flow resistance of the fluid flow towards the discharge nozzle 25.
The necessary amount of the cross-sectional area of the volute room A at the end point c is determined on the basis of the fluid flow rate and the fluid flow speed.
Therefore, as in the prior art, if a bulged portion 23a having a height gradually increasing in the direction from the winding start point b to the end point c is formed on the outer peripheral wall of the casing shell 23, the bulged height h at the end point c becomes excessively high in order to obtain a necessary cross-sectional area of the flow passage at the end point c.
In this arrangement, therefore, there is a problem that the so-called bulge forming using a press machine requires repeated pressing process together with annealing processes.
Further, the prior art has another problem in that it is necessary to arrange a guide 26 having a projecting shape at the end point c of the volute room A to decrease the flow resistance of the fluid flow towards the exhaust nozzle 25. Without such a guide 26, the pump performance will be decreased.